Non-linear optical crystal, such as lithium niobate or lithium tantalate single crystal, has a high secondary nonlinear optical constant. When a periodic domain polarization inversion structure is formed in the above crystals, a second-harmonic-generation (SHG) device of a quasi-phase-matched (QPM) system can be realized. Further, when a waveguide is formed within this periodic domain inversion structure, a high-efficiency SHG device can be realized and further, applied to wide range of applications such as optical communication, medical science, photochemistry, and various optical measurements.
In the case that an SHG device is produced for example, it is necessary to form an optical waveguide inside of the device and to optically polish end faces, to which a fundamental wave is made incident and from which a harmonic wave is emitted. In patent document 1, the applicant disclosed that, after the SHG device is cut out, many devices are laminated in the direction thickness to obtain a laminated body and the end faces of the laminated devices are subjected to optical polishing. Further, according to the patent document 1, it is disclosed to prevent pyroelectricity during the optical polishing.